This invention relates to the manufacture of polycrystalline silicon and more particularly to the growth of semiconductor grade polycrystalline silicon.
In the manufacture of silicon for the semiconductor industry, polycrystalline silicon is referred to as polysilicon and this latter term will be generally used herein. Polysilicon for the semiconductor industry is most generally produced by a thermal decomposition and deposition process wherein a silicon-containing composition, generally a silane, is reduced to effect deposition of polysilicon on a wire or rod. Such a polysilicon process, (or "poly" process) while of a general epitaxial nature, is to be distinguished from the semiconductor industry "epitaxial" or "epi" process wherein a silane, typically silicon tetrachloride (SiCl.sub.4) is decomposed and deposited to produce a monocrystalline layer of silicon on a monocrystalline wafer or substrate. To an extent, a polysilicon process will produce some monocrystalline silicon in the ingots but that is not the intent since the ingots are to be melted and pulled or zoned into monocrystalline silicon in a crystal puller or zoner.
In pulling monocrystalline silicon by the Czochralski method, pieces of polysilicon are placed in a crucible within a crystal pulling apparatus and melted. A monocrystalline piece (a seed) of silicon is then brought into contact with the surface of the melt so that material from the melt is caused to solidify on the seed. As the seed is raised away from the melt, material continues to "grow" thereon and a monocrystalline silicon rod thereby produced. In the zoning method of producing monocrystalline silicon, an ingot of polysilicon is mounted within the zoning apparatus. A seed is contacted to an end of the ingot and localized heat is used to melt the end of the ingot and the end of the seed. As material from the ingot solidifies on the seed, the source of localized heat and hence the molten zone is moved along the ingot leaving monocrystalline silicon where the molten zone had been. By analogy to the Czochralski method, the ingot of polysilicon ma be considered as the crucible and material used in the former method.
Polysilicon for the Czochralski method has been generally produced by deposition on a wire or a rod; typical material is tungsten or tantallium wire or silicon rod. This conductive member, if not silicon, is removed from the polysilicon ingot, as by drilling, prior to placement of the silicon in the puller crucible. (For zoning purposes, the polysilicon is often grown on starting rods of polycrystalline silicon, called slimrods, to distinguish them from the finished rods or ingots of monocrystalline silicon produced by the Czochralski or zoning methods. The slimrods of silicon need no be removed from the ingot prior to placement in the zoner.) When growing polysilicon, the starting deposition member is mounted in ambient controllable apparatus, such as bell jar reactor apparatus, with electrical connections made to the ends of the member so that it may be heated by an electric current. In most cases, the apparatus is single-ended, i.e., the feed-throughs for the electrical connections are through the base on which the bell jar is mounted. Thus, while in its simplest form the equipment may utilize but a single wire or rod, it is customary to use pairs of members connected in a "hairpin" or inverted "U" arrangement. Thus the lower end of each member of the pair is connected to an electrical source and electrical continuity completed by electrical connections between the top ends of the members.
The methods herein described for multirods also apply to the growth of monocrystal silicon on slimrods as well as polycrystal silicon on slimrods and the descriptions are equally applicable for both cases.
Further, pairs of deposition members have been included in the reactor for reasons of economy and productivity. Thus, arrangements of four to ten members have been used. The placement of the rods must be to provide adequate gas inlet for each rod, as well as provision for removal of unused and by-product gases. The rods must be spaced from each other and from the bell jar to allow for the ultimate diameter size of the ingot being grown. Within these criteria, a two pair arrangement has the members on the corners of a square, a three pair arrangement on the points of a hexagon, and a four pair arrangement on the points of an octagon, etc., for as many rods as can be accommodated. Thus, essentially all arrangements are a single ring or circle which allows sufficient spacing between the rods to allow the rods to grow to full diameter during the process.
The normal system for maintaining the rods at silicon deposition temperatures involves connecting them electrically in series or parallel to maintain uniform rod temperatures, and hence, relatively uniform growth. When the number of rods is more than four and the typical diameter to be grown is up to nine inches, much of the heat radiated from the rods is lost from the process, particularly when the rods are yet small. As the rods grow in diameter, the cross radiation from rod to rod becomes more effective as utilizing this heat within the system, particularly that heat which is radiated inside the ring of rods to the other rods. At the start of deposition and until the end is reached, there is a relatively low heat utilization, thus the reason for the large quantities of energy required to produce the silicon.